In the control of hydraulic fluids, it has been common it utilize a spool valve wherein the spool is yieldingly urged to a predetermined position and one or more solenoids are provided to which a constant current supply is supplied in order to move the spool and control the hydraulic fluid.
One of the problems with respect to such a construction is that there is no control of the rate at which the sliding spool shifts and as a result pressure and flow shocks occur in the hydraulic system.
This is due in part to the fact that a standard solenoid has a force characteristic that increases as the spool approaches its full stroke position which in turn acts to accelerate the shifting action of the sliding spool.
It has been common to use a force controlled solenoid which provides a linear adjustable force by altering the current to the solenoid which in turn creates a force balance against the resisting spring for the purpose of proportionally controlling a spool position relative to its overall stroke.
Accordingly among the objectives of the present invention are to provide a method of decelerating the shifting action with a sliding spool hydraulic directional valve resulting in lesser amount of pressure and flow shock in the hydraulic system.
In accordance with the invention, it has been found that if the force controlled solenoid is energized by a constant current providing a constant force which is resisted by a variable force provided by the opposing spring in the solenoid valve, a resultant increasing resistant force level is achieved as the spring is compressed throughout the range of travel. This increasing force differential actions to slow the axial velocity of the spool so that the opening of the valve occurs at a slower rate.
In accordance with the invention, a method of controlling spool shift speed in a solenoid operated hydraulic directional valve which includes a body, a spool slidably mounted in a bore in the body and operable to control hydraulic fluid flow, a force controlled solenoid adapted to be electrically energized by a constant current supply and a spring yieldingly urging the valve spool to a predetermined position, which method comprises applying a constant force to the spool valve which opposes the variable force of the spring thereby decelerating the shifting action of the spool.